Fix coverage reporting to include coverage by doc tests

[smangelsdorf]

In many cases, we've got a rustdoc example being used in lieu of a separate test case. These aren't being included by our current test coverage reporting via Travis / Coveralls.

[ELD]

Unfortunately, I don't believe that is possible. Based on the contents of this readme, it appears that the rustdoc example are created in a temporary directory that's disposed of when the destructor of the TempDir struct is run.

I also took a look at the master branch to see if this has changed but it appears to be the same. I don't know if there are plans to make the rustdoc executable remain after test runs or not.

[smangelsdorf]

That is unfortunate. Thanks for looking into this. We'll have to park this until the necessary support exists.

[bradleybeddoes]

Following #85 it appears that coverage reporting has been broken completely.

I am guessing this due to our use of cargo test -all now and cargo workspaces.

Investigation required, I wasn't able to sort it in the 30 minutes I spent on it earlier.